Power semiconductor die are conventionally attached to a metalized side of a substrate at the backside of the die by a soldered, diffusion soldered or sintered area joint. Electrical connections are typically made to the front side of the die facing away from the substrate by heavy Al-wire bonding or heavy Cu-wire bonding. Double-sided area joints have been used for example in double-sided cooling assemblies and from older thyristor and rectifier module designs, where metal clips are joined to the front side by soldering. The clip soldering method is also used in some discrete low voltage power MOSFET (metal-oxide-semiconductor field-effect transistor) packages. In so-called planar interconnect technology, the structure has an insulating layer on the carriers/substrates and a conductor pattern on top of the insulating layer. In other double-sided area joint approaches, a flexible board is used to provide an area contact to the front side of the die instead of a wire bond connection.
In each case, conventional area contacts suffer from a mismatch in the thermal expansion of the metal that is joined to the die and the semiconductor material. The mismatch causes thermo-mechanical stress in the joint partners and in the interface material. In any case high stress at the end of the edges and corners of the area joints causes delamination. With soldered area-contacts, the stress cracks the solder layer during power cycling or thermal cycling. The crack propagates from the edge to the center within the solder. In some planar interconnect technologies, copper is grown directly on the die metallization which is Al, AlSi or AlCu or AlCuSi. An interface metal layer is usually applied below the copper layer to be grown. The interface layer is rather thin and in the range of a few 100 nm. The weak layer for delamination is the Al or Al alloy die metallization. The cracks start at the edges and corners at the Al surface and moves toward the center within the Al die metallization layer. In the case of a sintered connection to the die Al die metallization, the crack starts at the edges and corners at the surface of the sintered layer and moves down into the Al die metallization layer and stays in the Al metallization propagating toward the center. Even though some conventional planar interconnect technologies have eliminated the weak soft solder in the area contact with the die, the lifetime of these structures is limited by this area joint because of the use of standard Al die metallization which is the weakest point in the area joint region.